one-shot operation with Flow in Android

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I'm trying to show a user information in DetailActivity. So, I request a data and get a data for the user from server. but in this case, the return type is Flow<User>. Let me show you the following code.

ServiceApi.kt

@GET("endpoint")
suspend fun getUser(@Query("id") id: Int): Response<User>

Repository.kt

fun getUser(id: Int): Flow<User> = flow<User> {
    val userResponse = api.getUser(id = id)
    if (userResponse.isSuccessful) {
        val user = userResponse.body()
        emit(user)
    }
}
.flowOn(Dispatchers.IO)
.catch { // send error }

DetailViewModel.kt

class DetailViewModel(
    private val repository : Repository
) {
    val uiState: StateFlow<User> = repository.getUser(id = 369).stateIn(
        scope = viewModelScope,
        started = SharingStarted.WhileSubscribed(5000),
        initialValue = User() // empty user
    )
}

DetailActivity.kt

class DetailActivity: AppCompatActivity() {
    ....
    initObersevers() {
        lifecycleScope.launch {
            // i used the `flowWithLifecycle` because the data is just a single object.
            viewModel.uiState.flowWithLifecycle(lifecycle, Lifecycle.State.STARTED).collect { state ->
              // show data
            }
        }
    }
    ...
}

But, all of sudden, I just realized that this process is just an one-shot operation and thought i can use suspend function and return User in Repository.kt.

So, i changed the Repository.kt.

Repository.kt(changed)

suspend fun getUser(id: Int): User {
    val userResponse = api.getUser(id = id)
    return if(userResponse.isSuccessful) {
        response.body()
    } else {
        User() // empty user
    }
}

And in DetailViewModel, i want to convert the User into StateFlow<User> because of observing from DetailActivity and I'm going to use it the same way as before by using flowWithLifecycle.

the concept is... i thought it's just one single data and i dind't need to use Flow in Repository. because it's not several items like List.

is this way correct or not??

kotlin-flow android-lifecycle android
1 Answers

Yeap, this one-time flow doesn't make any sense - it emits only once and that's it.

You've got two different ways. First - is to create a state flow in your repo and emit there any values each time you're doing your GET request. This flow will be exposed to the use case and VM levels. I would say that it leads to more difficult error handling (I'm not fond of this way, but these things are always arguable, haha), but it also has some pros like caching, you can always show/get the previous results.

Second way is to leave your request as a simple suspend function which sends a request, returns the result of it back to your VM (skipping error handling here to be simple):

val userFlow: Flow<User>
    get() = _userFlow

private val _userFlow = MutableStateFlow(User())

fun getUser() = launch(viewModelScope) {
    _userFlow.value = repository.getUser()
}

This kind of implementation doesn't provide any cache out of scope of this VM's lifecycle, but it's easy to test and use. So it's not like there is only one "the-coolest-way-to-do-it", it's rather a question what suits you more for your needs.

user_195803550
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